The present invention relates to implantable stimulation devices, e.g., cochlear prosthesis used to electrically stimulate the auditory nerve, and more particularly to an electrode array designed to provide focused electrical stimulation along the entire length of the cochlea (i.e., from base to apex,) and a method of insertion of the electrode array into the cochlea providing a snug fit against the modiolus.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. These people are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, numerous cochlear implant systemsxe2x80x94or cochlear prosthesisxe2x80x94have been developed which seek to bypass the hair cells in the cochlear by presenting electrical stimulation to the auditory nerve fibers directly, leading to the perception of sound in the brain and an at least partial restoration of hearing function. The common denominator in most of these cochlear prosthesis systems has been the implantation, into the cochlea, of electrodes which are responsive to a suitable external source of electrical stimuli and which electrodes are intended to transmit those stimuli to the ganglion cells and thereby to the auditory nerve fibers.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. In addition to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis perform the function of separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each channel of information would be conveyed selectively to the subset of auditory nerve cells that normally transmitted information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex, and ideally the entire length of the cochlea would be stimulated to provide a full frequency range of hearing. In practice, this ideal is not achieved because of the anatomy of the cochlea which decreases in diameter from the base to the apex, and exhibits variations between patients. Because of these difficulties, known electrodes can only be promoted to the second turn of the cochlea at best.
Over the past several years, a consensus has generally emerged that the scala tympani, one of the three parallel ducts that, in parallel, make up the spiral-shaped cochlea, provides the best location for implantation of an electrode array used with a cochlear prosthesis. The electrode array to be implanted in this site typically consists of a thin, elongated, flexible carrier containing several longitudinally disposed and separately connected stimulating electrode contacts, perhaps 6-30 in number. Such electrode array is pushed or inserted into the scala tympani duct to a depth of about 20-30 mm via a surgical opening made in the round window at the basal end of the duct. During use, electrical current is passed into the fluids and tissues immediately surrounding the individual electrical contacts in order to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone, or modiolus, adjacent to the scala tympani on the inside wall of its spiral course. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one contact site tends to activate selectively those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site. Thus, there is a need for the electrode contacts to be positioned as close to the ganglion cells as possible. This means, in practice, that the electrode array, after implant, should preferably hug the modiolar wall, and that the individual electrode contacts of the electrode array should be positioned on or near that surface of the electrode array which is closest to the modiolar wall.
In order to address the above need, it is known in the art to make an intracochlear electrode array that includes a spiral-shaped resilient carrier which generally has a natural spiral shape so that it better conforms to the shape of the scala tympani. See, e.g., U.S. Pat. No. 4,819,647. The ""647 U.S. patent is incorporated herein by reference.
It is also known in the art, as shown in applicant""s prior patents, U.S. Pat. Nos. 5,545,219 and 5,645,585, to construct an electrode carrier from two initially straight members, a rod-like electrode carrier and a flexible rod-like positioning member. As shown in these patents, the two members extend in substantially parallel relation to and closely alongside each other, but are connected to each other only at their respective leading and trailing end regions. After implant, a pushing force is applied to the positioning member so that it is forced to assume an outwardly arched configuration relative to the electrode carrier, thereby forcing the electrode carrier into a close hugging engagement with the modiolus, thereby placing the electrode contacts of the electrodes in as close a juxtaposition to the cells of the spiral ganglion as possible. The ""219 and ""585 U.S. patents are also incorporated herein by reference.
Unfortunately, while the electrode array taught in the above-referenced xe2x88x92219 and ""585 patents has the right idea, i.e., to force the electrode carrier into a close hugging engagement with the modiolus, it does so only through application of an additional pushing force which is applied to an electrode structure after it is already fully inserted into the cochlea. Such additional pushing force may easily cause damage to the delicate scala tympani. Moreover, the entire electrode array may twist during the insertion process, or when the additional pushing force is applied, thereby causing the electrode contacts to twist and/or be forced away from the modiolus, rather than in a hugging relationship therewith.
It is thus seen that improvements are still needed in cochlear electrodes, particularly to provide an electrode array that extends the length of the cochlea and that better assures that the electrode contacts assume a close hugging relationship with the modiolus.
The present invention addresses the above and other needs by providing an implantable cochlear electrode array and leading section that permits the electrode array to first be gently promoted through the entire length of the cochlea, thereby providing stimulation of the entire range of nerves, and then permits the electrode array to be gently pulled from both ends to position the electrode array against the modiolus , or the inner wall of the scala tympani, to better stimulate the nerve fibers.
In accordance with one aspect of the invention, there is provided a leading section that may be easily promoted through the entire length of the cochlea. The leading section contains no electrical components and is not a permanent part of the implant. After the leading section is in place it is used to gently pull the electrode array through the cochlea.
It is thus a feature of the present invention to provide a method of positioning the electrode array against the inner wall of the scala tympani. After pulling the electrode array through the scala tympani, the electrode array is positioned by gently pulling from both ends causing the electrode array to hug the inner wall of the scala tympani.
It is a further feature of the invention to provide wedging positioners that are inserted into each cochleostomy to fix the position of the electrode array following the insertion and positioning of the electrode array.
It is another feature of the invention to provide a method of electrode array insertion that advantageously requires only a gentle pulling force to properly position the electrode array, thus minimizing potential damage to the scala tympani.
In accordance with a first embodiment of the invention, the electrode array and leading section are manufactured as a single electrode assembly. The forward end of the leading section is inserted through a cochleostomy at the basal end of the cochlea and promoted through the cochlea. The forward end of leading section is then grabbed by a micro hook inserted through a second cochleostomy at the apical end of the cochlea and the forward end is pulled through the second cochleostomy. The leading section is then gently pulled through the second cochleostomy and the electrode array is fed through the first cochleostomy. When the electrode array is in the correct position within the cochlea, the ends of the electrode assembly emerging through the cochleostomies are gently pulled to seat the electrode array against the modiolus (or synonymously against the inner wall of the scala tympani.) Wedges are inserted at each cochleostomy to hold the electrode array in place, and the leading section is severed from the electrode array.
In accordance with an alternate embodiment of the invention, the electrode array and the leading section are separate detachable pieces that connect to form the electrode assembly. Additionally, the alternative leading section has an attaching end with a hoop, and the alternative electrode array has a forward end with a hook.
In the second embodiment the attaching end is fed through the apical cochleostomy. When the attaching end the reaches the basal cochleostomy, it is extracted using a micro hook. Then the hook on the electrode array is attached to the loop on the leading section, and the leading section is withdrawn pulling the electrode array into position within the cochlea. The electrode array is then gently pulled as in the first embodiment to seat the electrode array against the modiolus, i.e., against the inner wall of the scala tympani. Wedges are inserted at each cochleostomy to hold the electrode array in place, and the leading section is detached and discarded.
It is an additional feature that an non-tapered electrode array may be used in accordance with the present invention. When such non-tapered electrode array is used, the insertion process may be from either end of the cochlea.